In nonvolatile memory devices, multiple insulating layers such as tunnel insulating layers, charge trapping insulating layers and blocking insulating layers can be used to form a gate insulating layer. The charge trapping insulating layer generally includes silicon nitride which serves to trap charges. For example, charges can be injected into the charge trapping insulating layer by Fouler-Nordheim tunneling or hot carrier injection to form a stored logical value in a nonvolatile memory device. The stored charge is erased by discharging electrons from the charge trapping insulating layer or by injecting holes into the charge trapping insulating layer.
A multi-bit (multiple bit value) memory device can have an increased storage capacity without physically enlarging the size of a unit cell relative to that of a single bit memory device. For example, a 2-bit nonvolatile memory device can be formed by storing a single bit on each side of the charge trapping insulating layer. A 2-bit nonvolatile memory device may also be formed by storing single bits on respective sidewalls of a trench region in a semiconductor substrate in an attempt to address possible short channel effects.